1. Field of the Invention
The present invention relates to a positioning stage device that can be used for a semiconductor exposure device, for example, and driven with very high accuracy over a large area.
2. Description of the Related Art
In semiconductor exposure devices, and especially in step-and-repeat exposure devices (steppers), which are currently the mainstream, light (exposure light), which has passed through an original plate (hereinafter, reticle), on which a pattern desired to be exposed is drawn, is reduced by a constant ratio through a projection optical system (a lens), such that a sensitized agent (photoresist) applied on a base plate (hereinafter, wafer) is exposed. Thereby, the reticle pattern is transferred onto the wafer while the wafer, as a material to be exposed, remains positioned at a predetermined place. This process is repeated over the whole wafer surface. Step-and-scan exposure devices (scanners) are used for exposing a wider area by synchronously scanning a wafer and a reticle, whereas steppers keep the wafer still and perform one-shot exposure.
A positioning stage device for conveying a wafer (hereinafter, wafer stage) must be driven to the desired location on a two-dimensional plane (an XY plane) with high accuracy and over a large area. This is because higher accuracy is increasingly required as the feature size of semiconductor circuits is reduced. Also, because the region covered by a wafer stage drive must be large in order to accommodate the increasing wafer diameter, driving must be performed (i) to a replacement position for replacing the wafers, (ii) for the stage when a mark exposed on the wafer is measured at a place other than the exposure place, etc.
For detecting the position of the wafer stage, a laser interferometer is primarily used. By two-dimensionally arranging laser interferometers, the position of the wafer stage in two dimensions may be performed anytime. That is, as shown in FIG. 2, which is a schematic diagram of the main portion of a wafer stage device capable of moving in a two-dimensional plane according to the related art, a plane mirror for performing X-axis measurement (hereinafter, bar mirror) is mounted to a wafer stage 1 so as to extend in the Y-axis direction. An X laser interferometer 5 for measuring the position of the wafer stage 1 in the X-axis direction detects a relative drive amount of the wafer stage 1 by irradiating a laser beam in parallel with the X-axis onto an X-bar mirror 3 and causing the incoming reflected light to interfere with reference light. Position measurement of the wafer stage 1 in the Y-axis direction is performed in the same manner. By positioning either one of or both of the laser interferometer 5 and a laser interferometer 4 for the X- and Y-axes, the rotational angle θ around the Z-axis of the wafer stage 1 also can be detected.
On the basis of the positional information obtained by the laser interferometers 5 and 4, by arranging actuators (not shown in the figures), such as linear motors, etc., in two dimensions, the wafer stage 1 can be driven to a predetermined position.
When the wafer stage 1 is driven in the X-axis direction, the drive amount in the X-axis direction can be measured by the X laser interferometer 5 and the position in the Y-axis direction can be measured by the Y laser interferometer 4, for measuring a different place on the Y bar mirror. Therefore, problems arise in that the measurable area for the X and Y positions of the wafer stage is determined by the lengths of the X-bar mirror 3 and the Y-bar mirror 2 and that the measurement accuracy is affected by the surface machining accuracy of the bar mirrors 3 and 2.
In order to improve the positioning accuracy of a wafer stage, the machining accuracy of the bar mirrors should be very high. However, as described above, the larger the drive area of the wafer stage, the longer the length required for the bar mirrors 3 and 2. It would be very difficult to machine on the order of nanometers the entire surface of such a long bar mirror.
Accordingly, a method for providing a new laser interferometer is proposed in order to measure a position such as a wafer replacement position or a wafer mark measurement position when driving of a wafer stage over long distances is required.
FIG. 3 is a schematic diagram of major parts of a wafer stage device according to the related art, which is provided with plural laser interferometers for performing stage position measurement, which are arranged in the same direction, which can be switched. In FIG. 3, reference numerals identical to those in FIG. 2 indicate the same components. Referring to FIG. 3, the two-dimensional position of a wafer stage 1 is measured by an X1 laser interferometer 5a and a Y laser interferometer 4. When the wafer stage 1 must move over a long distance in the Y-direction for replacing a wafer, the extended position can be measured by an X2 laser interferometer 5b, installed in parallel with the X1 laser interferometer 5a, and separated by a distance L. Accordingly, by installing laser interferometers separated by some distance, the wafer stage 1 can be driven over a distance longer than the X-bar mirror 3.
Since a laser interferometer is used for measuring the relative displacement, the position cannot be correctly measured unless the reflected light returns each time. That is, the measurement cannot continue unless the reflected laser beam leaving the bar mirror returns. When the reflected light strikes the bar mirror and returns, the laser interferometer requires resetting, and, therefore, the X- and Y-axis interferometers are reset using absolute sensors.
According to the arrangement in FIG. 3, for example, photo switches (not shown) can be arranged at predetermined positions on the X- and Y-axes within an area wherein the position of the wafer stage 1 can be measured by the X1 laser interferometer 5a and the Y laser interferometer 4, and the instant that the wafer stage 1 crosses the predetermined position, the X1 laser interferometer 5a and the Y laser interferometer 4 are reset. Afterward, when the reflected light returns to the laser interferometers 5a and 4, the two-dimensional position of the wafer stage 1 is measured by the X1 laser interferometer 5a and the Y laser interferometer 4.
When the wafer stage 1 is driven to a wafer replacement position, for example, and leaves the measurement area of the X1 laser interferometer 5a, X-position measurement of the wafer stage 1 is performed by the X2 laser interferometer 5b. At that time, an area where the X1 laser interferometer 5a and the X2 laser interferometer 5b can perform position measurement at the same time can be assured by making the distance L between the X1 laser interferometer 5a and the X2 laser interferometer 5b be less than the length of the X-bar mirror 3. When the laser interferometers 5a and 5b for performing X-position measurement are switched, the wafer stage 1 is driven to a place wherein both the X1 laser interferometer 5a and the X2 laser interferometer 5b can perform measurement. This may be performed when the measured value of the Y laser interferometer 4 reaches a predetermined value. Alternatively, installing another sensor is allowed. At this point, the X2 laser interferometer 5b is reset. Then, by handing over the value of the X1 laser interferometer 5a to the X2 laser interferometer 5b, the X1 laser interferometer 5a can be switched to the X2 laser interferometer 5b, regardless of the X position of the wafer stage 1. Afterward, even if the X1 laser interferometer 5a cannot be used for measurement, the X position of the wafer stage 1 can be measured by the X2 laser interferometer 5b, and the wafer stage 1 can perform as if it is freely driven over a distance longer than the X-bar mirror 3. When the X2 laser interferometer 5b is switched to the X1 laser interferometer 5a, the same process can be applied.
As described in the related art, for a wafer stage requiring long distance driving in the Y-direction, by installing plural X laser interferometers (e.g., X1 and X2 laser interferometers) and by switching between them, the wafer stage can be driven for a distance longer than the X-bar mirror 3. When the X1 laser interferometer is switched to the X2 laser interferometer for performing X-axis position measurement, it is preferable that the wafer stage be moved to a place where the X1 and X2 laser interferometers can perform measurement at the same time, the X2 laser interferometer be reset, and the measured value of the X1 laser interferometer be handed over to the X2 laser interferometer.
However, in practice, the bar mirror is not in a perfect plane and has some dispersion arising from the surface machining accuracy. This is called a deterministic element. Accordingly, the error varies according to the place where the laser beam strikes.
FIG. 4 is a drawing for explaining positional measurement errors of a wafer stage device when switching between plural laser interferometers according to the related art. In FIG. 4, the error generated according to the planar shape of the X-bar mirror 3 is a function of the Y-position and is expressed by formula (1):Δx=F(y)  (1)where, Δx is the error in the X-direction and y is the Y-position of the stage.
Assuming that the X1 laser interferometer 5a measures at position Y=0 and the X2 laser interferometer 5b is separated from the X1 laser interferometer 5a by a distance L, the errors of the X1-interferometer 5a and the X2 laser interferometer 5b are respectively expressed as in formula (2):Δx1=F(y)Δx2=F(y+L)  (2)
Accordingly, position measurements made by the X1 interferometer 5a and the X2 laser interferometer 5b have an error difference of (Δx1−Δx2), and if the measured value of the X1 laser interferometer 5a is directly handed over to the X2 laser interferometer 5b, that quantity will be added to the error. This may be allowed when the accuracy is high enough, such as when replacing wafers; however, when switching laser interferometers within the exposure region or when performing high-accuracy mark measurement (alignment) outside the exposure region using an off-axis scope, for example, the error (Δx1−Δx2) associated with such switching cannot be ignored.
Also, when the value of a laser interferometer is inaccurate (includes an error) due to the effect of air fluctuations, for example, prediction will be impossible, since this case is indeterminate. Further, if it is assumed that the X1 laser interferometer 5a has an error of Δx(1), the instant that the X1 laser interferometer 5a is switched to the X2 laser interferometer 5b, the error of the X1 laser interferometer 5a is handed over to the X2 laser interferometer 5b. On the contrary, if it is assumed that the X2 laser interferometer 5b has an error of Δx(2) when switching to the X1 laser interferometer 5a, the error of the X1 laser interferometer 5a will be Δx(1)+Δx(2), and accordingly, the error will be accumulated whenever the laser interferometers are switched over.